Kino: A Rust-powered Ractor web server for Ruby 4.0

Kino is a new web server built for Ruby 4.0+ that leverages Ractors instead of forked processes. Since Ruby threads can't execute Ruby code in parallel due to the GVL, traditional servers fork a process per core, multiplying memory usage. Kino instead runs a Rust (tokio + hyper) network front-end that dispatches requests to parallel Ractors executing a Rack 3 app inside a single small process, with a threaded fallback mode for apps like Rails that aren't yet Ractor-shareable.

On a real 8-core server, every Kino mode outperforms a Puma fork cluster by 1.5-2x on I/O-light endpoints, and Ractor mode beats it by over 30% on pure CPU work. Memory savings are even more notable: roughly 7x less than a Puma cluster on a simple synthetic benchmark app, and about 4x less on a real Rails app running in threaded fallback mode.

The server ships with production-grade features out of the box: graceful drain, crash supervision and respawn, bounded queues with 503 backpressure, request timeouts, hardened intake against slowloris and TLS-handshake attacks, TLS via rustls, and live stats and logging. A built-in kino --check command diagnoses exactly what prevents an app from running in Ractor mode, and its config and CLI mirror Puma's familiar workers x threads model. Ractors remain officially experimental in Ruby 4.0, and Kino positions itself as the best way to experiment with them today while its threaded mode is already considered solid.